The present invention relates to the filtering of responses received by an extractor located downline with respect to a secondary radar receiver.
It is known that a radar can be fitted out with a device called a secondary radar that can be used to obtain, from cooperating carrier vehicles equipped with radar responders, coded information elements on the identity of the carrier and other coded information elements (altitude, reporting of radio malfunctions, distress signals etc).
The responders of the carrier vehicles can send out responses whenever they are interrogated and, sometimes, in one mode of operation known as the S mode, they can do so spontaneously. Each radar equipped with a secondary radar should therefore be provided with means enabling it to eliminate those responses, from among all the responses received, that are not responses to its own interrogations. The device that fulfils this function is a defruiter, the role of which is to get rid of the fruits, which is the name given to the responses that are made to other radar interrogators but are nevertheless received by all the radars.
In a known way, a defruiter comprises a synchronous filter which ascertains that the responses received during the listening periods that follow each of the interrogations are synchronous with the interrogations of the radar. The interrogation and the listening period that follows it constitute what shall hereinafter be called a recurrence. The radar making interrogations with a certain frequency of recurrence must receive responses that are time-lagged, the time lags of one antenna dwelling having the same value. The testing of the synchronism of the reponses may be such that each response is kept at its arrival during, for example, an interrogation period that may be extended to take account of the tolerance values proper to the responders and of the relative motions of the interrogator and of the responder, and is compared with the following interrogation. This comparison of the synchronism of two responses, the stored response and the following response, represents the simplest method, and the corresponding defruiter is called a "two out of two" defruiter. When the density of the fruits is high, it may happen that a fruit occurs, by chance, at the same period as an expected response and therefore constitutes, with the first response, a so-called "ghost" response that does not correspond to a real response.
Since this possibility has a non-negligible probability of occurrence, stricter criteria of acceptance have been created, corresponding to three out of three responses or four out of four responses. To consider that there is a synchronous response, the "three out of three responses" criterion requires that the following two successive responses should correspond in time with the first response received. The requirement is of three successive synchronous responses in the case of the "four out of four" criterion.
If the interrogator radar were to receive a response for each of the interrogations sent, the use of such criteria would only reduce the probability of keeping a fruit and hence of forming a ghost response.
For various reasons, such as the deactivation of the responder to prevent an overload, or the masking of the antenna, certain responses are not received and consequently, the defruiter contributes to lowering the quality of the information. This deterioration of the information is shown in figure la.
This figure is formed by rows of vertical lines.
The first row "a" represents twenty vertical lines numbered 1 to 20, representing 20 successive interrogations of an interrogator in a given direction.
The next two rows b and c represent the number of responses obtained at output of a defruiter assuming that 100% of responses are received, i.e. one response for each interrogation.
The row b is obtained with a 2 out of 2 criterion. At output of the defruiter, there are therefore nineteen right responses since the defruiter gives a response only after at least two responses have been obtained. This type of defruiter, by construction, loses the first response.
The row c is obtained with a 3 out of 3 criterion and here there are only 18 right responses since it is necessary to wait for the third response for the defruiter to give a response. There is also a systematic loss of two responses.
In the row d, dots are used to represent gaps in the responses, assuming that only 80% of the interrogations give rise to a reception of a response. We have therefore shown four randomly distributed gaps by the elimination, in the row d, of the responses corresponding to the interrogations 3, 8, 11, 15.
The rows f and g represent reponses at output of a defruiter working with a 2/2 or 3/3 criterion. The defruiter working according to a 2/2 criterion gives no more than 11 right responses and the defruiter working according to the 3/3 criterion gives no more than 6 right responses. It must be noted that what has just been related with respect to examples is verified whatever the criterion of correlation adopted, when this criterion is of the type with at least r right responses on s recurrences. The number r of correlating responses, i.e. responses synchronous with interrogations, in the same distance window, and the number s of recurrences considered to establish the correlation are often programmable to be constantly adapted to the density of the traffic and hence of the responses.
Because of the principle of correlation adopted, right responses are eliminated, i.e. responses that are liable to participate successfully in a successful correlation.
This elimination arises out of the fact that each response that arrives is kept for a number s of recurrences and that the operation will count on the following s-1 recurrences if there are at least r-1 synchronous responses.
The s-1 recurrences preceding the current response are therefore routinely lost.
The invention seeks to improve the probability of detection of the responses. According to the invention, a search is made, among all the sequences of s consecutive recurrences containing the current recurrence, to see if there are (r-1) other synchronous responses. Hence, not only the preceding s-1 recurrences but also the following s-1 recurrences are taken into account.
It is stressed that what is done here is not merely to set up a correlation with the responses of the s-1 preceding recurrences and with the responses of the s-1 following recurrences, i.e. a total of two correlations instead of the single correlation of the known methods. What is done is to verify that there is at least one correlation with the responses of the s recurrences preceding or following the current recurrence and containing it. This point is illustrated in FIG. 1b for an r/4 criterion. The row a of this figure shows a sequence of seven interrogations comprising a so-called current interrogation numbered 0, three preceding interrogations numbered -1 to -3 and three following interrogations numbered 1 to 3. The responses shown in rows b bear corresponding numbers. They are represented by a line when a response has been detected and by a dot when there has been no response. According to the invention, four correlations will be made with the four consecutive recurrences containing the current recurrence. These sequences of recurrences are represented by arrows numbered 1 to 4 in the FIG. 1b.
The first sequence is formed by recurrences -3 to 0, the second by recurrences -2 to 1, the third by recurrences -1 to 2 and finally the last one by the recurrences 0 to 3.
In the example shown in FIG. 1b, if a 2/4 (r=2) criterion is taken, the current response correlates in each of the four sequences. On the contrary, with a 3/4 criterion, only the third sequence gives a positive correlation result. A simple correlation done with the previous recurrences only or the following recurrences only, i.e. the sequences 1 and 4 in the example, would have led to rejecting the current response. The processing according to the invention therefore really contributes to increasing the probability of detection. Returning to the example, of FIG. 1a we have shown the responses that are correlating and are therefore transmitted in the case of a 2/2 criterion of correlation on the row h and in the case of a 3/3 criterion of correlation on the row i, when the responses are those indicated in the row d. To say that there has been correlation, and to set up the row h, not only the following recurrence but also the preceding recurrence has been considered for each response of the row d. Similarly, for the row i, not only the two following recurrences but also the two preceding recurrences have been considered for each response pulse of the row d. For these two rows (h and i), the results have been shown with only two correlations, wherein the sequences used for correlation contain the current reccurence in the first position and the last position respectively. The rows h and d are similar. This means that, with this new criterion, there has been, in this case, no deterioration of the information. This is not always the case. Thus, in the row i, there are 11 responses transmitted instead of 6 in the row g, for the same 3/3 criterion of correlation. This improvement of the result is possible because the responses considered as being synchronous are not only those giving a successful correlation with the following two recurrences but also those giving a successful correlation with the preceding two recurrences.
Through the invention, it is furthermore possible to obtain an improvement, for a same criterion of correlation, in the number of responses transmitted and hence to eliminate the fruits without any deterioration of the probability of detecting the synchronous responses.
It has indeed been seen that the principle of correlation based on the analysis of the future (or the past) of the response to be correlated is defective for the last (or first) responses of a transponder which, although synchronous, are considered to be fruits for they no longer have (or do not yet have) a sufficient number of other correlating responses. They are therefore eliminated, with a corresponding reduction in the probability of detection and even the precision of the radar (the mean value of the noise is computed for fewer responses).
The responses held back, as synchronous responses, by the defruiter according to the invention, are each of the synchronous responses that have participated in a successful correlation. The correlation is done for each of the s sequences of s recurrences containing the current response. The row j of FIG. 1a shows, for a defruiter working according to this principle, the synchronous responses transmitted after defruiting, the responses received being always those shown in row d. For each response from this row, three correlations have been made as explained with reference to FIG. 1b. The responses represented in the row j are those that have taken part in at least one successful correlation according to the 3/3 criterion. With reference to the row i, which is the best one for this correlation, there is an additional response which is the response No. 13. This response is not shown in the row i, for the upline responses 11 and downline responses 15 are missing. By contrast, the response 13 participates with a successful correlation with the upline response 12 and the downline response 14. It is therefore included in the row j.
It is known that the secondary radars can send their interrogations according to several modes of interrogation. The responses from the transponder are normally sent in the same mode. In the method according to the invention, it is possible to make correlations on recurrences without considering their mode. These will be intermode correlations. It is also possible to choose only the recurrences where the interrogation has the same mode. These will be monomode recurrences.
In a preferred embodiment, the correlations will be done both on groups of intermode sequences and on groups of monomode sequences, the correlation being declared to be positive if it has been successful on only one of the sequences.